WHAT IS VIRTUAL MACHINE?
is a completely separate individual operating system installation on your usual operating system. It is implemented by software emulation and hardware virtualization.
Virtual machine is a software implementation of a physical machine - computer - that works and executes analogically to it. Virtual machines are divided in two categories based on their use and correspondence to real machine: system virtual machines and process virtual machines. First category provides a complete system platform that executes complete operating system, second one will run a single program.
Frequently multiple virtual machines with their own OS's are used in server consolidation, where different services are run in separate virtual environments, but on the same physical machine.
The main advantages of virtual machines:
Multiple OS environments can exist simultaneously on the same machine, isolated from each other;
- Virtual machine can offer an instruction set architecture that differs from real computer's;
- Easy maintenance, application provisioning, availability and convenient recovery.
The main disadvantages:
It is not required that guest OS's are the same, it is possible to run Microsoft Windows and Linux or older versions of the OS on the same computer. The support of different Os's is rather popular in embedded systems: a typical use is the support of real-time OS simultaneously with a high-level OS (e.g. Linux, Windows).
- Virtual machine is not that efficient as a real one when accessing the hardware;
- When multiple virtual machines are simultaneously running on a host computer, each virtual machine may introduce an unstable performance, which depends on the workload on the system by other running virtual machines.